1. Field of the Invention
The present invention relates to a power source apparatus, more particularly, to a technology for reducing ripples contained in the output voltage.
2. Description of the Related Art
Power sources for digital IC are required to supply a low-voltage high-current at a high throughput rate and to retain the fluctuations of an output voltage within a very narrow range. However, in general power source systems, a high-capacity smoothing capacitor is necessary at the output side to satisfy those requirements. Furthermore, in a low-voltage high-current environment, despite the fact that the effect of impedance components in the circuit becomes considerably large, presently available technology fails to produce parts with sufficiently low impedance. For this reason, in order to inhibit the decrease in efficiency, increase in output voltage ripples, increase in switching surges, and the like caused by the impedance components, many power sources have to be implemented as multiphase configurations and an output smoothing capacitor has to be connected in parallel, resulting in a larger number of parts, increased cost, and enlarged size of the circuits.
In a system suggested as effective means for resolving the above-mentioned problems, a DC voltage is supplied, in principle, without an output smoothing capacitor in a low-voltage high-current region by using two insulated step-down inverters, inducing rectangular waveform voltages with a phase difference of 180° in the secondary windings of the inverters, half-wave rectifying and superimposing the voltages.
Such a system was disclosed, for example, in JP-A-2003-102175. Thus, this system comprises a pair of switching elements, which are connected by one terminal thereof to both terminals of primary windings of at least two output transformers connected by neutral points thereof to one pole of an input DC power source, and by other terminal to the other pole of the input DC power source, means for controlling this pair of switching elements that one of them is non-conductive if the other is conductive, and rectifying means connected to each secondary winding of each aforementioned output transformer, wherein the conducting timing of each switching element is so set that the output voltage values of the rectifying means are equal to each other, and ripples of one rectified output waveform are covered by the other rectified output waveforms.
Further, U.S. Pat. No. 5,008,759 discloses a two-phase converter similar to that of the aforementioned patent reference. This two-phase converter employs means to synthesize the transformer outputs with an 180° phase difference by controlling the charging timing from a power supply source to a capacitance element and discharging timing from the capacitance elements to each transformer by using equivalently four switching elements.
The above-described power source apparatuses were useful because ripples contained in the output voltage were very small, the output smoothing circuit was unnecessary in principle, and the apparatuses could be small, lightweight, and highly efficient. However, all the above-described approaches required four switching elements at a primary winding side of the output transformer.